EP2591741A1

EP2591741A1 - Polyaxial bone anchoring device

Info

Publication number: EP2591741A1
Application number: EP12192588.7A
Authority: EP
Inventors: Lutz Biedermann; Wilfried Matthis; Bernd Fischer
Original assignee: Biedermann Technologies GmbH and Co KG
Current assignee: Biedermann Technologies GmbH and Co KG
Priority date: 2011-11-14
Filing date: 2012-11-14
Publication date: 2013-05-15
Anticipated expiration: 2032-11-14
Also published as: ES2536720T3; US20150313646A1; CN103099661B; US9017390B2; US20130123861A1; EP2591738A1; KR20130054157A; CN103099661A; JP2013103133A; US9333017B2; EP2591741B1

Abstract

A polyaxial bone anchoring device is provided including a bone anchoring element (1) having a shank (2) for anchoring in the bone and a head (3); a head receiving part (5,5') having a first end (5a) and an open second end (5b) a central axis (C) extending through the first end (5a) and the second end (5b) and a hollow interior portion (56) in communication with the open second end for receiving the head (3) therein; a locking ring (6) mounted around the head receiving part, the locking ring having a rod receiving portion (63,64); a cap (8,8') configured to cover the rod and to engage the head receiving part wherein the cap (8,8') is rotatable relative to the head receiving part (5) between a first position in which the legs are out of engagement with the head receiving part and a second position in which the legs engage the head receiving part and wherein upon tightening of the locking element the rod and the bone anchoring element (1) are locked with respect to each other.

Description

The invention relates to a polyaxial bone anchoring device including a bone anchoring element for anchoring in the bone or a vertebra, a head receiving part for receiving a head of the bone anchoring element, a locking ring mounted around the head receiving part for coupling a stabilization rod to the head receiving part and to the bone anchoring element and a cap with a locking element for securing the rod and for locking the whole assembly. When the rod is mounted in a rod receiving portion of the locking ring and the cap placed onto the assembly, the cap is rotatable with respect to the head receiving part from a first position in which it is out of engagement with the head receiving part to a second position in which engages the head receiving part. The locking element exerts pressure onto the rod to move the locking ring in a position where it locks the head in the head receiving part. Simultaneously, the rod is fixed by the locking element.
US 2009/0149887 A1 describes an apparatus for connecting a bone anchor to a support rod including a connector body, a sleeve around the connector body and a cap.
US 2011/0213419 A1 describes a screw and rod fixation assembly including a body member including a screw seat for seating a screw head therein and a rod seat for seating a rod therein. An adjustable locking ring applies a locking force against the screw head seated in the screw seat and a locking cap adjustably applies a locking force against a rod seated in the rod seat. The adjustable screw locking ring and locking cap function independent of each other.
It is the object of the invention to provide a polyaxial bone anchoring device having an enlarged field of applications, in particular being suitable for applications that require bone anchors with small size.
The object is solved by a polyaxial bone anchoring device according to claim 1. Further developments are given in the dependent claims.
The polyaxial bone anchoring device makes use of a cap and a locking element received in the cap for securing and fixing the rod and the head. Therefore, a problem of splaying of portions of the receiving part when tightening the locking element as known from conventional bone anchors does not occur. Furthermore, with the cap, the bone anchoring device has a low profile in axial or height direction of the bone anchoring element. This renders the bone anchoring device particularly suitable for use in the cervical spine or in pediatric applications, trauma and minimally open applications or for using in bone surgery.
With the polyaxial bone anchoring device, a modular bone anchoring system can be provided. The modular system includes the head receiving part preassembled with the locking ring and the cap preassembled with the locking element and a set of various bone anchoring elements having different shanks. Various shanks with different diameter, thread form or other different features can be combined with a head receiving part and locking ring according to the requirements in a particular clinical situation. Therefore, the surgeon has a substantial choice of implants. By the modularity, the costs of stock-holding can be decreased.
Further features and advantages will become apparent from the description of embodiments by means of accompanying drawings.
In the drawings:

Fig. 1:: shown an exploded perspective view of the polyaxial bone anchoring device according to a first embodiment.
Fig. 2:: shows a perspective view of the polyaxial bone anchoring device of Fig. 1 in an assembled state.
Fig. 3:: shows a perspective view from the top of the head receiving part of the polyaxial bone anchoring device according to the first embodiment.
Fig. 4: shows a perspective view from the bottom of the head receiving part according to the first embodiment.
Fig. 5:: shows a cross-sectional view of the head receiving part according to the first embodiment wherein the section has been taken perpendicular to the rod axis.
Fig. 6:: shows a top view of the head receiving part according to the first embodiment.
Fig. 7:: shows a perspective view from the top of the cap of the polyaxial bone anchoring device according to the first embodiment seen from the top.
Fig. 8:: shows a perspective view from the bottom of the cap according to the first embodiment.
Fig. 9:: shows a side view of the cap according to the first embodiment.
Fig. 10:: shows another side view, rotated by 90°, of the cap according to the first embodiment.
Fig. 11:: shows a top view of the cap according to the first embodiment.
Fig. 12:: shows a cross-sectional view of the cap according to the first embodiment along line A-A of Fig. 11.
Fig. 13:: shows a perspective view of the locking ring of the polyaxial bone anchoring device according to the first embodiment.
Fig. 14:: shows a cross-sectional view of the locking ring according to the first embodiment, the cross-section taken along line B-B in Fig. 13.
Fig. 15:: shows a top view of the locking ring according to the first embodiment.
Fig. 16a):: shows a cross-sectional view of the assembled polyaxial bone anchoring device, wherein the section has been taken in a plane perpendicular to the rod axis.
Fig. 16b):: shows an enlarged view of a portion of Fig. 16a).
Figs. 17 to 22:: show steps of assembly and use of the polyaxial bone anchoring device according to the first embodiment.
Fig. 23:: shows a perspective view of the polyaxial bone anchoring device according to a second embodiment.
Fig. 24:: shows a perspective view of the polyaxial bone anchoring device according to the second embodiment in an assembled state.
Fig. 25:: shows a perspective view from the top of the head receiving part of the polyaxial bone anchoring device according to the second embodiment.
Fig. 26:: shows a perspective view from the bottom of the head receiving part according to the second embodiment.
Fig. 27:: shows a top view of the head receiving part according to the second embodiment.
Fig. 28:: shows a cross-sectional view of the head receiving part according to the second embodiment, the cross-section being taken along line C-C in Fig. 27.
Fig. 29:: shows a perspective view of the cap of the polyaxial bone anchoring device according to the second embodiment seen from the top.
Fig. 30:: shows a perspective view from the bottom of the cap according to the second embodiment.
Fig. 31:: shows a side view of the cap according to the second embodiment.
Fig. 32:: shows a side view of the cap of the second embodiment, rotated by 90°.
Fig. 33:: shows a top view of the cap according to the second embodiment.
Fig. 34:: shows a cross-sectional view of the cap according to the second embodiment, the cross-section being taken along line D-D in Fig. 33.
Fig. 35:: shows a side view of the polyaxial bone anchoring device according to the second embodiment in an assembled state seen in a direction along the rod axis.
Fig. 36a):: shows a cross-sectional view of the polyaxial bone anchoring device according to the second embodiment shown in Fig. 35.
Fig. 36b):: shows an enlarged view of a portion of Fig. 36a).
Figs. 37a) and 37b):: show perspective views of a head receiving part of a polyaxial bone anchoring device according to a further alternate embodiment; and
Figs. 38a) to 38c):: show steps of assembly and use of the polyaxial bone anchoring device according to the further alternate embodiment.

As shown in Figs. 1 and 2, the polyaxial bone anchoring device according to a first embodiment comprises a bone anchoring element 1 in the form of a bone screw having a shank 2 with a threaded portion and a spherical segment-shaped head 3. The head 3 has a recess 4 (Fig. 16a)) for engagement with a tool. The bone anchoring device further includes a head receiving part 5 for receiving the head 3 of the bone anchoring element and a locking ring 6 for receiving a rod 7, for example, a spinal stabilization rod, and for connecting the rod 7 to the bone anchoring element 1. In addition, the bone anchoring device comprises a cap 8 for securing the rod 7 and a locking element 9 in the form of a set screw for locking the rod and the head 3.
Referring in particular to Figs. 3 to 6, the head receiving part 5 has a first end 5a and an opposite second end 5b, a central axis C and a coaxial through hole 5c. At the first end 5a the head receiving part has a cylindrical portion 50 with an outwardly extending flange or rim 51. A cylinder-segment-shaped recess 52 is formed at the first end 5a. The recess 52 serves as a guidance and seat for the rod 7. At both ends of the cylinder-segment-shaped recess 52 the rim 51 is flattened resulting in opposite flat surfaces 52a, 52b. At an angle to the cylinder axis L, there are cutouts 53a, 53b from the rim 51 so that two opposite rim portions 51 a, 51b extend outward in radial direction. The rim portions 51a, 51b extend in a circumferential direction substantially between the end of the rod guiding recess 52 up to an angle of around 90° to the cylinder axis L. A base surface of the cutouts 53a, 53b is cylindrical. The rim portions 51a, 51b each have on their side facing away from the first end 5 a a circumferentially extending undercut portion 51c. The undercut portion 51c is formed by a surface that is inclined outwards and towards the second 5b. The undercut portion 51c serves for engagement with a portion of the cap 8. The cylindrical portion 50 continues in a conically-shaped outer surface portion 55 widening towards the second end 5b. Further, an internal hollow spherical section 56 forming an accommodation space and a seat for spherical segment-shaped head 3 of the bone anchoring element 1 is formed in the head receiving part 5. The internal hollow spherical section 56 is configured to encompass the head of the bone anchoring element from the side covering a region including a largest diameter of the head 3.
A plurality of slits 57 are provided that are open to the second end 5b. The slits 57 extend substantially through the wall of the internal hollow portion 56 and render the head receiving part 5 flexible in the region where the head 3 is received. By the size and the number of the slits 57 a desired elasticity is provided to the head receiving part 5. The elasticity of the head receiving part 5 is such that the head 3 of the bone anchoring element 1 can be inserted by expanding the head receiving part and can be clamped by compressing the head receiving part.
Referring to Figs. 7 to 12, the cap 8 is a substantially cylindrical part with a first end 8a and an opposite second end 8b. At the first end 8a, there is a coaxial threaded through hole 81 for receiving the locking element 9. A coaxial bore 82 having a greater diameter than the threaded through hole 81 extends from the second end 8b to a distance from the first end 8a. The threaded through hole 81 opens into the coaxial bore 82. At a distance from the second end a circumferential groove 83 is provided that has a lower inclined wall. By means of this, the cap has adjacent its second end 8b an inwardly directed rim 85 with the inclinded wall forming an undercut portion 84. The inclination of the undercut portion 84 corresponds substantially to the inclination of the undercut portion 51c of the head receiving part 5.
The cap 8 further comprises two opposite substantially cuboid-shaped recesses 86a, 86b that extend from the second end 8b to a distance from the first end 8a. The recesses 86a, 86b have rounded edges. By means of the recesses 86a, 86b two opposite longitudinal legs 87a, 87b are formed in the lower portion of the cap. The recesses 86a, 86b have such a height and width that the cap 8 is configured to cover the rod 7 when the rod 7 is mounted to the head receiving part 5 and the locking ring 6. Further, the recesses 86a, 86b have a width in circumferential direction that is substantially greater than the diameter of the rod 7 so that the legs 87a, 87b are configured to fit into the space between the locking ring and the rim portions 51a, 51b at the position of the cutouts 53a, 53b. In other words, the legs 87a, 87b have a width in circumferential direction that is smaller or equal to the width of the cutouts53a, 53b at the head receiving part 5.
The locking element 9 in the form of a set screw is configured to be screwed into the threaded through hole 81. Although the thread is shown to be a flat thread, any other thread form such as a metric thread, can be used.
Referring to Figs. 13 to 16b), the locking ring 6 has a ring portion 60 with a first end 6a and an opposite second end 6b. The outer surface of the ring portion 60 may be slightly conical, tapering towards the second 6b, to reduce the size of the bone anchoring device in radial direction. The ring portion 60 has a curved internal surface portion 62, wherein the curvature is directed to the center of the locking ring. The curved surface portion 62 can have a spherical curvature, but other types of curvatures are possible. The inner diameter of the locking ring is such, that the locking ring 6 can slide along the outer conical surface portion 55 of the head receiving part 5, thereby compressing the head receiving part 5 increasingly when sliding downwards.
At the first end 6a, the locking ring 6 comprises two projections 63 located diametrically opposite to each other. The projections 63 have such a height that they project up and away from the first end 5a of the head receiving part, when the locking ring 6 is mounted around the head receiving part 5, as depicted, for example, in Fig. 18. At their free ends, the projections each have a recess 64, that has the shape of the segment of a circle including at least a portion with the greatest diameter. The diameter of the circular segment-shaped recess 64 corresponds substantially to the diameter of the rod 7. By means of the recess 64 each projection 63 comprises upstanding legs 63a, 63b, that allow the rod 7 to be catched therein. As seen in Figs. 13 and 15, the side 63c of the projections that faces the center of the ring is flat.
The locking ring 6 may comprise at the position of the projections 63 on the inside of the locking ring at its first end 6a two inwardly projecting surfaces 65. , Such surfaces may act as a stop.
The flexibility of the head receiving part 5 and the size of the head receiving part 5 of the open second end 5b allows to mount the locking ring 6 from the second end 5b. When the locking ring 6 is mounted onto the head receiving part 5, it is prevented from rotation around the head receiving part 5 due to the flat surfaces 52a, 52b of the head receiving part that are facing the flat sides 63c of the projections 63. The dimensions of the parts can be designed such, that the head 3 is preliminarily held in the head receiving part 5 by a slight friction force and the locking ring 6 also can be held by a friction force in a preliminary manner around the head receiving part 5.
As can be seen in Figs. 1, 3 and 16a) to 16b) the head receiving part, the locking ring and the cap are configured to allow a placement of the cap onto the locking ring with inserted rod in a first position, in which the legs 87a, 87b extend into the cutouts 53a, 53b of the head receiving part 5. The cap 8 and the head receiving part 5 are further configured to allow a rotation of the cap to a second position in which the undercut portion 84 of the cap engages the undercut portion 51c of the head receiving part. In the second position, the cap 8 is prevented from being pulled-off in a direction along the central axis C of the head receiving part.
The head receiving part 5, the locking ring 6, the cap 8 and the locking element 9 as well as the bone anchoring element 1 are made of a bio-compatible material, for example, of titanium or stainless steel or of bio-compatible alloys, such as nickel-titanium alloys, for example, Nitinol, or of a bio-compatible plastic material, such as, for example, polyetheretherketone (PEEK). The parts can be made all of the same or of different materials.
Referring to Figs. 17 to 22, steps of mounting and use of the polyaxial bone anchoring device will be explained. As shown in Fig. 17, the locking ring 6 is mounted from the second end 5b to the head receiving part 5 such that, as shown in Fig. 18, the projections 63 for receiving the rod 7 extend upwards from the first end 5a of the head receiving part 5. The orientation of the locking ring 6 is such that the projections 63 are aligned with the cylinder-segment shaped recess 52 of the head receiving part. The rim portion 51 at the position of the ends of the recess 52 is supported by the support surfaces 65 of the locking ring.
The head receiving part 5 and the locking ring 6 may be delivered in pre-assembled manner. In a second step, shown in Fig. 18, the bone anchoring element 1 with a suitable shank 2 for a specific clinical application is mounted from the second end 5b to the assembly formed by the head receiving part 5 and the locking ring 6. The locking ring is in its uppermost position. The head 3 is introduced into the internal hollow section 56 of the head receiving part 5. This is possible, because the head receiving part 5 is flexible.
At this stage, the modularity of the bone anchoring device allows to combine a specific bone anchoring element with a specific shank with the head receiving part during or before surgery. The polyaxial bone anchoring device thus assembled is then inserted into the bone or a vertebra. In an alternative way of use, the bone anchoring element is first inserted into the bone or into a vertebra and thereafter the assembly of head receiving part 5 and locking ring 6 is mounted to the head 3 after implantation of the screw.
Then, as shown in Fig. 19, the locking ring 6 is shifted downwards so that it compresses the head receiving part 5. The head 3 can be held in a preliminary angular position and the locking ring 6 may also held in its position by frictional forces acting between the head receiving part 5 and the head 3 on the one hand and the locking ring 6 and the head receiving part 5 on the other hand.
In a next step, as shown in Fig. 20, the rod 7 is inserted into the recess 64 provided at the projections 63. Because the recess 64 has a circular segment shape including more than a largest diameter, the rod can be catched in the recesses, so that it is preliminarily held in place therein.
In a next step, as shown in Fig. 21, the cap 8 is placed onto the assembly The locking element 9 may be pre-assembled with the cap 8. In the first position the legs 87a, 87b extend along the cutouts 53a, 53b. By means of this, the legs 87a, 87b extend into the space between the projections 63 of the locking ring and the rim portions 51a, 51b of the head receiving part 5. Next, as shown in Fig. 22, the cap 8 is rotated so that its undercut portion 84 engages the undercut portion 51c at the rim portions 51a, 51b of the head receiving part 5. In the second position, the cap 8 is prevented from being pulled away in an axial direction. Then, the locking element 9 is tightened. Its lower surface contacts the rod 7 and the rod 7 is pressed into the recess 64 of the projections 63 of the locking ring. The force acting onto the rod is transferred to the locking ring and the locking ring is shifted downwards until it firmly compresses the head receiving part and locks the head 3. Hence, the head and the rod are locked simultaneously. Also, when tightening the locking element, the head receiving part and the cap are twisted against each other and the whole assembly is fixed.
A second embodiment of a polyaxial bone anchoring device will be described with reference to Figs. 23 and 24. Parts and portions that are identical to the parts and portions of the first embodiment are designated with the same reference numerals and the description thereof shall not be repeated. The bone anchoring device according to the second embodiment differs from the bone anchoring device according to the first embodiment in the design of the head receiving part and the cap.
As shown in particular in Figs. 23 to 28, the head receiving part 5' lacks a rod guiding recess. It comprises a first cylindrical portion 50'and an outwardly extending rim 51' having two opposite radially and circumferentially extending brackets 51a', 51b' with upstanding rims having an undercut portion 51 c'. Each bracket 51 a', 51 b' has a flattened side so that opposite flat surfaces 52a', 52b'are formed at the outer circumference of the cylindrical portion 50'. Between the flat surfaces 52a', 52b' and the brackets 51a', 51b', there are cylindrical surfaces 53a', 53b' for allowing the legs of the cap 8 to be inserted between the brackets 51 a', 51b' and the flat surfaces 52a', 52b'. Furthermore, the head receiving part 5'comprises at its first end 5a an upstanding circular rim 54 with a diameter smaller than the outer diameter of the cylindrical portion 50'. At the outside of the rim 54, there is an inclined surface 54a with an inclination opposite to the inclination of the undercut portion 51' so that it forms together with the undercut portion 51c' of the brackets 51a', 51b' a substantially dovetail-shaped groove 5d as can be seen in particular in Fig. 28. The lower portion of the head receiving part 5' is identical or similar to that of the head receiving part of the first embodiment.
Referring to Figs. 29 to 34, the cap 8' according to the second embodiment differs from the cap according to the first embodiment in that it comprises a coaxial bore 82' that has a diameter only slightly larger than the diameter of the threaded through hole 81'. Cuboid-shaped recesses 86', 86b' result in longitudinally extending legs 87a', 87b' of the cap 8'. At the second end 8b the cap comprises dovetail-shaped portion 85' that cooperates with the dovetail-shaped groove 5d at the head receiving part 5. The polyaxial bone anchoring device according to the second embodiment has an even more reduced height.
Hence, when the cap 8' is placed on the assembly consisting of an anchoring element 1, head receiving part 5' and locking ring 6, in the first position, the legs 87a', 87b' extend along the cylindrical portions 53a', 53b' of the head receiving part. Then, as shown in Figs. 35b, 36a) and 36b), the cap 8' is rotated thereby engaging with the dovetail-shaped rim 85' the dovetail-shaped groove 5d at the head receiving part 5 so that the cap is connected to the head receiving part and the rod is secured against removal. The dovetail shape prevents pulling off of the cap in an axial direction. Then, the locking element is tightened to fix the whole assembly.
In Figs. 37a) and 37b), a head receiving part of a further alternate embodiment is shown. In Figs. 37a) and 37b), a head receiving part 5" is shown with a configuration different from the head receiving part from the first embodiment. Namely, the rim portions 51a and 51b have a different configuration, which can be described as being on opposite portions of the head receiving part 5", as compared to the rim portions of the head receiving part of the first embodiment.
In Figs. 38a) to 38c), a further alternate embodiment of a polyaxial bone anchoring device includes the head receiving part 5" shown in Figs. 37a) and 37b). The assembly of the bone anchoring device in Figs. 38a) to 38c) is similar to that shown in Figs. 20 to 22, with the exception that in Figs. 38a) to 38c), the cap 8 and the locking element 9 are tightened in the same direction, whereas in Figs. 20 to 22, the cap 8 and the locking element 9 are tightened in opposite directions. A portion of the locking ring 6 has a height in axial direction between the second end and the projections 63 that is greater than that of the locking ring according to the previous embodiments. Hence, the locking ring may encompass a portion of the legs of the cap 8. The alternate steps illustrated in Figs. 38a) to 38c) may follow the step shown in Fig. 19.
In Fig. 38a), the rod 7 is inserted into the recesses 64 provided at the projections 63. Because the recesses 64 have circular segment shapes including a largest diameter of the rod 7, the rod 7 can be caught or held in the recesses 64, so that it is preliminarily held in place therein.
In a next step, as shown in Fig. 38b), the cap 8 is placed onto the rest of the assembly. The locking element 9 may be pre-assembled with the cap 8. In the first position, the legs 87a, 87b extend along or are aligned with the cutouts 53a, 53b, and thus extend into spaces between the proj ections 63 of the locking ring 6 and the rim portions 51a, 51b of the head receiving part 5".
Next, as shown in Fig. 38c), the cap 8 is rotated so that undercut portions 84 engage undercut portions 51c at the rim portions 51a, 51b of the head receiving part 5". In this second position, the cap 8 is prevented from being pulled away from the rest of the assembly in an axial direction. Then, the locking element 9 is tightened in the same direction as the cap 8. For example, both locking element 9 and cap 8 may be rotated clockwise, or alternatively, both may be rotated counterclockwise, in order to be tightened or locked. The tightening of both the locking cap 8 and the locking element 9 in a same direction is enabled by the alternate arrangement of the cutouts 53a, 53b and the rim portions 51a, 51b. Namely, the cutouts 53a, 53b are oriented to be upstream of the rim portions 51 a, 51b, with respect to the direction of locking of both the cap 8 and the locking element 9. Because the cap 8 and the locking element 9 are tightened in the same rotational direction, the locking force of locking the assembly is increased. In addition, with a same direction of locking, tightening of the locking element 9 will not result in inadvertent loosening of cap 8.
Upon tightening, the lower surface of locking element 9 contacts the rod 7 and the rod 7 is pressed into the recesses 64 of the projections 63 of the locking ring 6. The force acting onto the rod 7 is transferred to the locking ring 6, and the locking ring 6 is shifted downwards to firmly compress the head receiving part 5" and lock the head 3 therein. The head 3 and the rod 7 may be locked simultaneously upon fixing or locking the assembly. When tightening the locking element 9, the connection between the head receiving part 5" and the cap 8 may also be reinforced.
It shall be understood that modifications of the embodiments shown are possible. Any engagement structure of the cap and the head receiving part that prevents pulling off of the cap in the engaged position is possible. In particular, the size of the rim portions 51a, 51b or the dovetail groove 5d in a circumferential direction may be different from the embodiments shown.
Other modifications are conceivable. The configuration of the locking ring and the cooperating outer surface portion of the head receiving part can be designed otherwise. For example, it is possible, that the inner surface of the locking ring is tapered and cooperates with the tapered outer surface of the head receiving part. Or, the outer surface portion of the head receiving part is convexly rounded and the inner surface of the locking ring is straight or tapered. The cooperating surfaces of the locking ring and the head receiving part can also be parallel so that the clamping of the head is achieved by an interference fit between the locking ring and the head receiving part.
For the bone anchoring element, various bone anchoring elements can be used that differ with respect to their shank length, diameter or thread form or that have a through channel with openings for the introduction of drugs or bone cement. Also, hooks, nails or any other anchoring elements are possible. The head of any such anchoring elements and of the anchoring elements shown in the embodiments described above has preferably a spherical outer surface portion and the head receiving part has a spherical set portion for the head.
The rod receiving recess can also be semi-circular, quarter-circular or U-shaped or can have any shape that is configured to accommodate a rod therein.

Claims

A polyaxial bone anchoring device, the polyaxial bone anchoring device including
a bone anchoring element (1) having a shank (2) for anchoring in the bone and a head (3);
a head receiving part (5, 5') having a first end (5a) and an open second end (5b) a central axis (C) extending through the first end (5a) and the second end (5b) and a hollow interior portion (56) in communication with the open second end for receiving the head (3) therein, the head receiving part being flexible so as to allow clamping of the head;
a locking ring (6) mounted around the head receiving part, the locking ring having a rod receiving portion (63, 64);
wherein the head (3) is pivotable in the head receiving part and can be locked at an angle by compressing the head receiving portion by means of the locking ring (6;
a cap (8, 8') configured to cover the rod and to engage the head receiving part and having a first end (8a) and a second hand (8b) and a central axis (C) extending through the ends and two opposite legs (87a, 87b; 87a', 87b') extending in the direction of the central axis;
a locking element (9) extending through the cap and configured to engage the rod;
wherein the cap (8, 8') is rotatable relative to the head receiving part (5) between a first position in which the legs are out of engagement with the head receiving part and a second position in which the legs engage the head receiving part and
wherein upon tightening of the locking element the rod and the bone anchoring element (1) are locked with respect to each other.
The bone anchoring device of claim 1, wherein the head receiving part (5, 5') comprises a first portion (51, 51') adjacent to the first end (5a) with an undercut (51c, 51c'; 54a) for engagement by the cap.
The bone anchoring device of one of claims 1 or 2, wherein the head receiving part comprises two 180° spaced apart radial projections (51a, 51b; 51a', 51') for engagement with the cap (8, 8').
The bone anchoring device of claim 3, wherein the radial proj ections (51a, 51b; 51a', 51a') are arranged substantially at 30° to 60° with respect to the rod axis (L) when the rod (7) is inserted.
The bone anchoring device of one of claims 1 to 4, wherein the head receiving part (5, 5') comprises a flexible portion with an outer surface (55) that has an increasing outer diameter towards the open end (5b).
The bone anchoring device of one of claims 1 to 5, wherein the head receiving part (5) comprises a plurality of slits (57) in its wall.
The bone anchoring device of one of claims 1 to 6, wherein the head receiving part (5, 5') has a through hole (5c) at the first end with a diameter smaller than a greatest diameter of the head (3).
The bone anchoring device of one of claims 1 to 7, wherein the locking ring (6) comprises two projections (63) opposite to each other in a circumferential direction, the projections each having a recess (64) configured to insert the rod and preliminarily hold the rod therein.
The bone anchoring device of claim 8, wherein the recess (64) has a circle segment contour, the segment being preferably greater than a semi circle.
The bone anchoring device of one of claims 1 to 9, wherein the cap is substantially cylindrical and comprises two circumferentially opposite recesses (86a, 86b; 86a', 86b') open to the second end for passing through the rod.
The bone anchoring device of one of claims 3 to 10, wherein the width of the legs (87a, 87b; 87a', 87b') in circumferential direction is smaller or equal to the distance between the rod (7) and the radial projections (51a, 51b; 51a', 51b') when the rod is in inserted.
The bone anchoring device of one of claims 1 to 11, wherein the cap (8) comprises a threaded through hole (81) and wherein the locking element (9) is a set screw provided in the through hole.
The bone anchoring device of one of claims 1 to 12, wherein the cap (8) has an undercut portion (85) for engagement with a portion (51c) of the head receiving part.
The bone anchoring device of one of claims 1 to 13, where in the head receiving part (5') has a dovetail groove (5d) and the legs (87a', 87b') have a dovetail portion (85') for engagement with the dovetail groove.
The bone anchoring device of claim 1 to 14, wherein the head receiving part (5) comprises a rod guiding recess (52) at the first end (5a).